Slashdot videos: Now with more Slashdot!

View

Discuss

Share

We've improved Slashdot's video section; now you can view our video interviews, product close-ups and site visits with all the usual Slashdot options to comment, share, etc. No more walled garden! It's a work in progress -- we hope you'll check it out (Learn more about the recent updates).

separsons writes "On May 18th, Japan's Aerospace Exploration Agency (JAXA) will launch Ikaros, a fuel-free spacecraft that relies completely on solar power. The spacecraft's 46-foot-wide sails are thinner than a human hair and lined with thin-film solar panels. After a rocket brings the craft to space, mission controllers on the ground will steer Ikaros by adjusting the sails' angles, ensuring optimal radiation is hitting the solar cells. If the mission proves successful, the $16M spacecraft will be the first solar sail-powered craft to enter deep space."

Sure, but 46 feet wide is a pretty small target in the vast, empty vacuum of space. Beefing up the thickness wouldn't make it tough enough to resist impacts at the velocities that space rocks would be hitting it at anyway. And if an object with mass hits your solar sail, probably it's better for it to punch clean through and impart as little kinetic energy into your vehicle as possible, so that it doesn't get knocked off course as badly.

Yes, you are right -- micrometeoroid impacts are definitely an issue that you have to deal with when you are using thin-membrane materials in space. Hopefully the engineers will design features called "rip stops" (among other names) into the sail to prevent tears from spreading through the sail. These are usually accomplished by putting a grid of perforations throughout the sail -- when a tear encounters one, the circular shape spreads the tensile stresses across the adjacent material, reducing the likelihood that the tear will propagate. This way a micrometeoroid impact won't ruin your entire sail, just the local grid element.

There are probably other methods of implementing rip stops, but I haven't read any significant literature on them. Anything bigger than a micrometeoroid, and you have bigger problems -- but in this case, a traditional satellite would have just as big a problem.

A solar panel collects light and turns it into electricity. And solar panels are much thicker than a human hair.

A solar panel is anything flat and, in this context, photovoltaic. The part of a typical solar panel where the magic happens is much thinner than a human hair; it's the junction between two materials. The rest of it is just there to protect that part (and to enable its production during manufacturing, of course.) But since thin-film solar panels have been around for more than a little while, you have no excuse for not knowing about them and yet simultaneously "contributing" to this discussion. Thin-film panels are now cost-competitive with crystalline panels and are expected to eventually be much less expensive due to their reduced energy cost of manufacture. This also reduces the time to energy payback, which was around seven years with crystalline panels in the 1970s. (I have GOT to find my source on that again, must be in some old homedir someplace...) And in space, nothing non-structural has to hold up its own weight or survive winds (aside from the solar wind) so it can be as thin as will provide sufficient tensile strength. Like, say, a sheet of plastic.

A solar sail converts photon impact to momentum. Anything photons are absorbed or reflected by is a solar sail. A solar panel converts a portion of photon impact to electricity, trading photon velocity for electron velocity. These are not incompatible goals. As far as I understand, a reflective solar sail actually imparts more velocity than an absorptive one, but both work.

I guess you didn't mean this, but just to avoid confusion. There is something called "solar wind", charged particles ejected from the sun, it has nothing to do with this sail. The sail uses light pressure, the pressure of light emitted by the sun.

Nope. Tilt the sail so there is thrust against the direction of orbital motion and the ship will fall inward toward the sun. Think of the spacecraft with the sail at 45 degrees to the radial direction ot the sun, so light is reflected along a tangent to orbit in the direction of motion.

So long as a solar sail craft is in orbit, it can either raise or lower its orbit more-or-less at will, although it is easier nearer the sun than further out. Once it is out of orbit, however, it can't ever return.

Not sure about that. I've seen claims that a lot of the thrust of a solar sail would be due to the solar wind...which would tend to stick, and thus couldn't be tacked against.

Those claims are wrong. The force on a solar sail due to solar radiation pressure is about a thousand times that of the solar wind.

Also, solar cells tend to absorb photons, capturing their momentum, and when they re-radiate it (at a lower frequency) the direction is random.

The solar cells are going to be absorbing a small fraction of the incoming photons. If the sail is designed properly, the rest will be reflected in a controllable direction.

If this is correct, then the simple model of solar sails tacking using reflected light is at least an oversimplification, and possibly so much of an oversimplification that it doesn't properly predict the effects.

Your assumptions are wrong, and the model is correct.

MESSENGER has used its mostly reflective solar panels to make deliberate course changes. The basic physical principle is already proven, not just in the lab, but in space. JAXA is examining the practicality of building large solar sails, not whether they will work at all.